1. Field of the Invention
The present invention relates to a technique, specifically apparatus and an accompanying method, for imparting direct resistance heating to a specific location in, for example, a conductive metallic specimen under test and which is particularly, though not exclusively, suited for use in adding an independent dynamic thermal capability to a mechanical test system. Advantageously, through use of the invention, axial thermal gradients that would otherwise appear along a gauge length of the specimen while it self-resistively heats can be set to a desired level or even substantially eliminated altogether, while specimen end sections, outside the gauge length, and grips, that hold opposing ends of the specimen, are not appreciably heated.
2. Description of the Prior Art
Metallic materials play an indispensable role as an essential component of an enormous number of different products and thus clearly occupy an important role in the world economy. As such, their properties and costs both to manufacture and utilize must be carefully controlled to maximize their utility and value. Doing so is commonly accomplished through tests and physical simulation of both metal manufacturing and fabrication processes and applications. More recently, computer simulation, relying on data gained through physical simulation, has been used in defining and selecting an appropriate metallic material for a given application in view of its requirements and the expected behavior, as reflected by that data, of various materials then under consideration.
Thermo-mechanical test systems currently exist which, under computer control, simultaneously treat metallic specimens to both thermal and mechanical pre-defined test procedures (commonly called “programs”) in order to accurately simulate various manufacturing and processing steps.
Within a physical simulation system, a mechanical system is used to controllably deform metallic test specimens. The deformation is often provided by servo-controlled hydraulic units or electric motor driven systems which can impart tensile, compressive or torsion forces on a specimen to controllably strain and/or deform it. Further, through the mechanical system, a tension, compression or torsion deformation program may be repeated several times on the same specimen and at different amounts and rates. Test specimens, used in deformation studies, typically fall within a variety of sizes and shapes depending on the type of mechanical test to which the specimen is to be subjected.
The specimen is held, near its ends, between two grips which together mechanically restrain the specimen and provide electrical and thermal contact with opposing ends of the specimen. Thermal heating current is routed through the grips. Depending on a test to be done, a resulting thermal gradient, occurring lengthwise through the specimen may be adjusted by changing grip materials and/or shape of the specimen, to create, on the one hand, a very steep gradient or, on the other hand, a very shallow gradient, or any desired value there between, or even no appreciable gradient at all.
In many types of tests that require specimen heating, it is desirable to have as uniform a temperature as possible across a region of the specimen which is of interest in the test methodology (this region being commonly referred to as a “gauge length”). However, owing to the elevated temperatures involved, the grips which are employed to hold the specimen at its ends typically need to be water cooled so that mechanical properties of the grips are not compromised at those temperatures. In addition, the end section of the specimen situated between the gauge length and each grip should be mechanically strong to prevent that section from deforming which, should that occur, will adversely affect the test results. Consequently, specimens are typically designed with a reduced diameter gauge length, as compared to the diameter of each end section, such that the gauge length will deform first when mechanical work is applied to the specimen.
One of three types of thermal systems is generally used in a physical simulation system: a furnace, an induction heater or self-resistive heating.
Furnaces, which rely on establishing convective heat currents from a heat source, often radiant, to a specimen surface and from there inward into the specimen, provide relatively slow heating rates. Two generally used methods exist for heating the specimen with a furnace. In one method, an entire specimen and its grips, with the specimen then being held by the grips, are fully inserted in the furnace. The furnace then heats both the specimen and the grips. Hence, the entire portion of the specimen lying between the grips is considered the gauge length. Through another method, a furnace is sized so that only a reduced diameter gauge length of the specimen is enclosed in the furnace but not the grips. The furnace is typically built with three internal heating zones traversed by the entire gauge length. Both end zones of the furnace are typically hotter than a center zone. This additional heating capability provided to ends of the gauge length compensates for specimen heat losses occurring to the grips as they hold the specimen. While a properly designed furnace system produces a uniform temperature throughout the gauge length, the available heating rate produced by the furnace is substantially slower than those needed for simulating many manufacturing and fabrication processes.
Induction heating, which imparts more localized specimen heating than attainable through a furnace, yields heating rates which are considerably faster than those typically provided by a furnace. Here, an induction coil is placed around and covers the specimen gauge length and heats the specimen material through high frequency induction. The coils of an induction heater are shaped to provide additional heating capability to the ends of the gauge length to compensate for heat losses to the grips. While this scheme tends to adequately function, acceptable results generally occur only after significant effort has been expended on a trial-and-error basis to properly position the induction coils relative to the gauge length. Thus, an operator often needs to have considerable skill in that regard to properly utilize induction heating. Further, the induction coils tend to create localized hot spots within the specimen gauge length. Consequently, a resulting temperature distribution along the gauge length will not be as uniform as would occur with a furnace.
Fastest yet is direct self-resistance heating mechanisms where heating current is passed directly through the specimen and the specimen self-resistively heats. While self-resistance heating is the most versatile, by virtue of its very high heating rates, and adaptable of the three heating techniques, it is typically the most difficult to control. Given its relatively fast heating rate, self-resistance heating can be used to reproduce thermal characteristics, in a metallic test specimen, which are inherent in a wide variety of metal manufacturing processes and applications.
Typically, in existing thermo-mechanical test systems, both electrical and mechanical connections are made to the same locations on the specimen, i.e., where the specimen is held at its opposing ends by a pair of grips. Consequently, self-resistive heating currents flow between the grips and end-to-end through the specimen. Each grip is typically water cooled or mounted in a water cooled jaw system to prevent that grip from overheating and being damaged during a test. Inasmuch as the current flow is substantially uniform across any specimen cross-section taken transverse to the axial current flow through the specimen, then essentially isothermal crosswise planes are established through the specimen. However, owing to its reduced cross-sectional area, the gauge length will have a higher electrical resistance than the specimen end sections. When the electrical current flow is high, as required to keep the specimen at relatively high temperature between a relatively cool jaw system, thermal currents flow primarily from a reduced diameter gauge length to larger-diameter specimen end sections. Consequently, in traditional direct resistance heating methods and as a result of applying the heating current, the specimen typically has its highest temperature at its midpoint and its coldest temperature at its ends where the mechanical grips are located, hence causing a thermal gradient to appear from the mid-point to each end of the specimen. While a steep thermal gradient is useful for some types of tests such as weld heat-affected zone simulations, it is not desirable in certain other types of tests such as thermal mechanical fatigue tests. To compensate for heat losses on the ends of the specimen and hence reduce the gradients, grips have been developed which exhibit reduced contact area with the specimen and thus produce reduced specimen cooling. While this approach creates a more uniform temperature distribution along the specimen length, it adversely affects various mechanical properties of both the grips and the specimen, particularly where the grips attach to the specimen which, in turn, prevents these grips from being used effectively for many types of tests. Furthermore and of significant consequence, since the current and mechanical attachment points are the same, grip designs which reduce the thermal gradients create a more uniform temperature over the entire length of the specimen and not just over the gauge length. By heating the specimen section between the gauge length and each grip to a similar temperature as the gauge length, that section is weakened which, in turn, allows unwanted deformation to occur in that section during a concurrent mechanical test program, thereby possibly distorting the results of that test program.
Currently, the GLEEBLE material testing systems manufactured by Dynamic Systems Inc. of Poestenkill, N.Y. (which also owns the registered trademark “GLEEBLE”) are dynamic thermo-mechanical material testing systems that utilize a computer-controlled servo-hydraulic mechanical system to controllably strain and/or deform a specimen along with a self-resistive heating system to controllably produce essentially cross-wise isothermal planes through the specimen, thus permitting both mechanical and thermal programs to be imparted to the specimen. In these systems, self-resistive heating is accomplished by using two electrically and thermally conductive jaw/anvil assemblies or other appropriate grip systems, each of which securely holds an opposing end of the specimen, with a reduced diameter work zone there between. Large flexible conductors provide a path for large electrical heating currents between the jaw/anvil assemblies and an electrical power supply. Each jaw/anvil assembly is electrically isolated from the remainder of the mechanical system to prevent electrical heating currents from flowing through the latter and bypassing the specimen. However, inasmuch as the heating current used in even these systems flows end-to-end through the specimen, specimen end sections adjacent to the gauge length may experience unwanted deformation during certain thermo-mechanical test programs.
Moreover, a majority of material testing systems currently in use, other than the GLEEBLE systems, and which have a servo-controlled hydraulic mechanical system, possess no inherent specimen heating capability. Such testing systems simply perform mechanical test programs on specimens which remain at room temperature. While appropriate and commercially available induction heating systems and furnaces can be added to these testing systems to provide some degree of specimen heating, neither of these modalities is as responsive or controllable, and hence as useful as a self-resistance heating system.
Accordingly, a need exists in the art for a technique, including apparatus and accompanying methods, which can be readily adapted to a conventional mechanical testing system in order to impart a much-needed thermal capability to self-resistively heat a metallic specimen under test and particularly to apply heating current along the gauge length but without appreciably heating the grips of specimen sections outside the gauge length and thus substantially prevent unwanted deformation. Preferably, the apparatus should not appreciably, if at all, alter any of the mechanical capabilities of a conventional testing system and should also be suitable for use in conventional thermo-mechanical test systems that already employ direct resistance specimen heating.